Typically, printed circuits are made by an additive, a semi-additive, or a subtractive process. In an additive process, a conductive pattern is deposited on a nonconductive substrate and thereafter the conductive pattern is built up such as by electroless and/or electrolytic deposition of the conductive metal. Many problems are especially associated with adhesion of the circuit pattern to the substrate. Coating the substrate surface with an adhesive primer helps to alleviate the problem. However, that causes shortcomings, for example, the primer remains on the surface after processing causing tackiness which is especially unwanted when packaging the material. In addition, the optical appearance of the film is also undesirable and may affect resist processing. Improper adhesion is also a cause for poor conductivity in a circuit path causing inadequate deposition of an electrolytically deposited metal.
In a semi-additive process, the circuit is formed by depositing, typically electrolessly, a thin layer of a conductive metal, then covering the future conductive and nonconductive areas with a photographically developable layer which is thereafter removed from the conductive areas of the circuit and the circuit is built-up electrolytically with a metal. Thereafter, the photo-resist (the layer covering the future nonconductive areas) is removed and the electroless metal is removed such as by etching, etc. A semi-additive process suffers from the disadvantages of the extra photo-developing steps, the high cost of photo polymers, the requirement for a thorough removal of the photo-resist so that the electrolytically built up metal does not delaminate as well as removal of the metal.
Additionally, a noble metal etch resist must be added for protection when the blanket conductor is removed. Other shortcomings also have an effect on the deposit. For example, if the thickness and evenness of the resist is not critically correct "pin holing," "mushrooming," and metal background shows up in open spaces. Still other disadvantages are etch residues, plus etchant degradation of the substrate surface affecting optical properties.
In a subtractive process, the future nonconductive area is part of a continuous and conductive layer or sheet on top of a substrate. A photo-resist is developed on the conductive area and the nonconductive area is free from photo-resist. The metal from the future nonconductive area is removed with etchants or by electrolytic back plating (as an anode).
Inasmuch as a considerable amount of a metal must be removed, the subtractive process is slow and the conductive patterns tend to be undercut, i.e., some removal of the metal takes place under the photo-resist. Consequently, adhesion of the circuit to the substrate is impaired, which is especially undesirable in flexible circuits. Moreover, when preparing base material for subtractive process applications either an adhesive is used or hot-roll lamination is practiced for joining the metal layer. In either case, the substrate surface is altered creating additional problems in subsequent photo optical processing.
With this brief background in mind, the following prior art has been considered in evaluating the present invention: U.S. Pat. Nos. 2,703,722; 2,917,439; 2,941,918; 3,006,819; 3,052,957; 3,146,125; 3,267,007; 3,562,038; 3,620,933; 3,625,758; 3,937,857; 3,954,570; 4,006,047; 4,096,043; 4,100,037; 4,144,118, 4,149,768; and 4,159,414.